Improved Estimation of the Gross Primary Production of Europe by Considering the Spatial and Temporal Changes in Photosynthetic Capacity from 2001 to 2016

Round 1

Reviewer 1 Report

I have enjoyed reading the manuscript entitled “Improved Estimation of the Gross Primary Production for Europe by Considering the Spatial and Temporal Changes in Photosynthetic Capacity from 2001 to 2016” that was submitted to Remote Sensing. This study applied the FGM model to improve GPP estimation by introducing a spatially and temporally explicit Vcmax derived from satellite-based LCC at 41 EC sites and the regional scale. There is seldom research has focus on such topic, so the current study is on a topic of relevance and general interest to the readers of the journal. Based on my personal evaluation, there are still some minor issues should be improved before publication.

Offering more information about the method in the abstract. For example, “At the regional scale, when neglecting changes in Vcmax, the FGM overestimated the annual GPP by 0.5 to 2.9 Pg C yr-1 or 5 to 31%....” How do the authors evaluate the regional results?

This work calibrated the parameter using 21 sites, and the remaining 19 sites were reserved for independent validation. How do the authors separate the sites?

More details should be added in the manuscript. Besides, better to check the writing of the full text. Results need to have more specific data descriptions to support the conclusion of this study. Some related references are recommended in following section. https://doi.org/10.1016/j.rse.2022.112896. https://doi.org/10.1016/j.jhydrol.2022.128833

Some specific suggestions:

L33: “R2”, explain the abbreviation.

L34: “RMSE”, explain the abbreviation.

L149: “Some fundamental equations for GPP estimation in the FGM are described here.”, Move to L148.

L132: Please unify the descriptions of EC site and MCD12Q1 in the Figure 1, such as modify “Mixed forest to “MF”

L181: “… estimated from Beer’s law as follows:.”, Move to L180

L199: Based on the data sharing policy of Flux2015, please add related references for each site.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The authors of the manuscript validated the Farquhar GPP model (FGM), compared the performance of the FGM with other models and evaluated the impact of Vcmax change on GPP across Europe. The theme addressed in the manuscript is very important, since there are several models for estimating the GPP that are applicable to some types of coverage, which makes it difficult to study the GPP on a large scale. The manuscript is well written and discussed, but I would recommend some tweaks.

All justification of the introduction is based on the FGM. However, there are GPP products and GPP estimation models, whose performances were also evaluated in this manuscript. As such, I recommend that authors contextualize these GPP models and products in the introduction.

Define Ci in the introduction.

I recommend that the authors add a data analysis subsection in the material and methods section, where it is described how the performance of the models was analyzed, and how the studied models were intercompared. Although regression is widely used in model validation, I personally think it is misused, since the calculation of the regression equation is used to study cause and effect. A scatter plot is important, as used by the authors, but without establishing a regression equation. The authors could evaluate the correlation coefficient (r) and not the equation determination coefficient (R2). By the way, they defined R2 as correlation, and you need to correct this misconception.

Author Response

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Author Response File: Author Response.docx